Why is this?

It's called blind ignorance.

Logical fallacy? What logical fallacy?

Recombination of gene elements as a means of expanding genetic divesity within a species is well documented in the literature. The issue is not diversity, the issue with just two specimens is accumulated mutations being expressed. Adam and Eve would not have any accumulated mutations! The only thing your question demonstrates is you really don't understand genetics in living systems, but rather still adhere to the discreditied selfish gene model.

Two people is plenty. The entire population of elephant seals came back from a single pregnant cow. They are thriving, and if you DNA test any two elephant seals they read as siblings.

We know there is at least one point in human history in which, although the population was not down to two people, there are single persons who is related to everyone alive to day. Any two humans on earth will DNA test as no further than 50th cousins. That's science.

So the concept that two people would not have the genetic diversity to spawn the human race is BS. Especially when your trying to argue that we spawned from a common ancestor 4 billion years ago.

So how did the initial "whatever" you think was the start gain your genetic diversity?

Now you can "solve the obvious and blatant logical fallacy."

Well actually , all the different races descended from Noah's sons , Shem, Ham, and Japheth.Also ,Adam and Eve's children had children and so on and so forth , so all Adam and Eve really had to do was have the first generation and it would continue .

They have no logical explanation for any claim they make so the answer is always "Goddidit"!

They are desperate to make up an excuse for a serious flaw in their fairy tale.

*Magic*